For the geological disposal of radioactive waste, there is provided an artificial barrier to prevent radionuclides from leaking out toward humans'living environment. Examination has been made on the use of an artificial multi-barrier wherein a low radionuclide-diffusion layer consisting of cement-based material functionally expectable to suppress nuclide diffusion and a low water-permeability layer having as its main component sodium-type bentonite expectable to provide swelling and cutoff are used in combination as shown in FIG. 1, as artificial barrier constituents in subsurface-disposal treatment facilities.
To evaluate the long-term stability of artificial barriers of this type, it becomes necessary to comprehend the change in quality of each artificial barrier constituent as well as the interaction between the barrier constituents. Especially, the leaching-out of calcium ions from the cement-based material causes not only the change in quality such as the conversion of sodium-type bentonite to the calcium type but also the resultant reduced performances at the same time as the decline in quality of cement-based material itself, and therefore, its earnest studies are in progress (Refer to Non-patent documents such as 1 to 3 and 5 mentioned infra, for instance).
The use of concrete permitting less formation of causal calcium hydroxide of such leaching-out from the cement-based material is considered as one measure against the above leaching-out. Heretofore, it has been definitely shown that the use of fly ash as a substitute for part of cement makes it possible to suppress the leaching-out.
Various studies have been also made on the influence of the action of solutions having bicarbonate ions and/or the phenomenon of the conversion of cement-based material to the densified form due to its neutralization on the physical properties of the cement-based material. Non-patent document 4 describes that bicarbonate ions occurring in the underground water are made to react with calcium hydroxide (calcium ions) leaching out from concrete so that a calcite (calcium carbonate) membrane is formed on a concrete surface, and consequently, the suppression of the subsequent leaching-out is attainable with this membrane.
As regards the change in quality of bentonite, Patent document 1 describes a bentonite quality change-proof agent containing metallic salts subjected to change into hydroxide under the alkali environment, in connection with an agent for use in waste disposal fields and/or low-level radioactive waste treatment facilities to prevent the change in quality of bentonite and a bentonite water-impermeability structure involving the use of this agent.
Patent document 2 describes that when using bentonite slurry for concrete joint cutoff and/or construction of a water impermeability layer in radioactive waste geological disposal fields or the like, bentonite slurry given in a slurried form with an aqueous solution of sodium bicarbonate, sodium carbonate or the like is adapted to prevent the change in quality and ensure the fluidity of bentonite slurry with the smaller amount of water as compared with the absence of these elements, and consequently makes it possible to maintain the bentonite swelling and cutoff.
Patent document 3 describes that as regards a cement composition used for grout material or the like having as its main component cement-based material with 0.1 to 0.5% by weight of bentonite, it is possible to maintain the bentonite swelling by adding sodium carbonate or the like.
Patent document 4 describes a waste treatment facility ground-creation process comprising forming a cutoff layer around waste without the need for any large-scaled apparatus, wherein bentonite-based material consisting of interlayer cations of bentonite and interlayer-cation water carrying the same type of interlayer cations is sprayed through nozzles to provide the cutoff layer. In this document, aqueous solutions of sodium carbonate, potassium carbonate, iron carbonate, sodium bicarbonate and potassium bicarbonate or the like are given as available examples of the interlayer-cation water.
The invention described in this Patent document 4 is to provide the execution of works by means of spraying with the small amount of water and maintain the bentonite swelling and cutoff by adding an aqueous solution of carbonate or bicarbonate to the bentonite-based material, like the invention described in Patent document 2.